L2: Basic Biomechanical Factors, Concepts and Terminology

We think of the musculoskeletal system (MSK) as…

a series of simple machines that connect and allow movement to occur

What are machines used for?

Are used to increase mechanical advantage (MA)

What are the 4 functions of machines?

1) They can be put together so there is a balance of multiple forces

2) Enhance force production

• reducing the total F needed to overcome a R

3) Enhance ROM and speed of movement

• so R can move further or faster than applied F

4) Machine can be set up so when F is applied to an object it moves in the opposite direction of the applied force

• pushing (applied F) someone (R) forwards and they move sideways

What are the 3 machines in the human body?

The arrangement of the MSK allows for 3 machines

1) Levers (most common)

2) Wheels-axels (often in ball-and-socket joints)

3) Pulleys

How do humans move (machines)

Humans move through a lever system

Can lever systems be changed? How can they be improved?

Lever systems cannot anatomically be changed, however they can be used more efficiently (increasing MA)

Define: Lever

a rigid bar that turns about an axis of rotation (AoR)

Define: Axis

Point rotation about which a lever moves to some degree

• also know as fulcrum

How do levers rotate around an axis?

Levers rotate around an axis due to the application of F (also known as effort) to cause its movement against a R.

In the Body

• Rigid Bar=

• Axes=

• Force=

Bones are the rigid bar. The bone being moved should always be identified first.

Joints are the axes. What joint is the bone being moved connected to?

Muscles contract to apply F, and it is the F that moves the bar around the A.

What are the three points that determine the type of lever?

The location of the three points determine the type of lever and the motion it is suited for.

Axis (A) / Fulcrum: The point of rotation

Force (F): point of force application

Resistance (R): point of resistance application

• Either the center of gravity of the segment or the location of external resistance.

What are the three lever systems?

First Class Lever: A is between the F and R

Second Class Lever: R is between the A and F

Third Class Lever: F is between the A and R

What is a force arm (FA)? What is a resistance arm (RA)?

FA: distance between A and F

RA: distance between A and R

General FA and RA length for levers

1st Class Lever: FA and RA length are generally equal

2nd Class Lever: FA is greater than RA length because R is always in the center

3rd Class Lever: RA length is greater than FA length because F is always in the center

What are the three machine-like functions of 1st class levers?

Depends on how far/close A is to the F and R

1) Producing balanced movements: When A is in the middle and R and F are at an equal distance

• Balancing opposing forces

2) Produce speed and ROM: When A is closer to F

3) Produce force motion: When A is closer to R

• enhanced F to move a relatively large R

FCL: Neck flexion and extension

Flexion

A: atlanto-occipital joint

F: Face muscles

R: Center of gravity of the back of the head

Extension

A: atlanto-occipital joint

F: Neck muscles

R: Center of gravity of face

FCL: Elbow extension

Elbow extension with shoulder fully flexed and arm beside the ear, the triceps applies force to the olecranon of ulna behind the axis of elbow joint.

As the applied F exceeds the amount of forearm R, the elbow extends S

Second Class Lever: One machine-like function is

1) Enhances force production: relatively less force is required to move a larger R

SCL: Human ex.

Plantarflexion of foot to raise the body on the toes.

A = Ball of the foot

F: Plantar flexors applying force to calcaneus

R: to lift the weight of the body

Third Class Lever: One machine-like function

1) To produce speed and ROM movements

• This is how we are built, we require a great amount F to move a small R

TCL: Biceps Brachii in elbow flexion

Using the elbow joint (A), the biceps brachii applies force at its insertion on radial tuberosity (F) to rotate the forearm up, with its center of gravity (R) serving as the point of resistance application.

Define: Force

something that pushes and pulls on the body/object

What is the force in the body???

Muscles push and pull on bones

What are the 4 components of force?

1) Magnitude: How big is the force pushing and pulling (on the bone)

2) Direction: Which way is force pushing and pulling (the bone)

3) Point of application: Where is the force applied on the body. (How far is the F from the A that the bone is rotating around)

4) Line of action: From the point of application we create a vector that is equal and opposite of the point of F, direction, and magnitude

How are forces generally represented?

Forces are generally represented with an arrow (vector) to show ALL 4 characteristics

When can we envision a movement arm? What is a moment arm? What does it help us with?

Once we have all 4 characteristics, we can envision a moment arm.

The moment arm is the perpendicular distance from an AoR to a line of action.

The moment arm is an IMAGINARY line, but it helps us understand MA and how it leans into machine-like functions.

What is torque?

Torque (moment of force), is the turning effect of an eccentric force

What makes up torque?

Force (of the muscle) x Force arm (aka moment arm of muscle) = joint torque output

What is a centric force?

Line of action (F), passes directly through the A.

• Causes: Linear translation

What is eccentric force?

Force applied in a direction that is NOT aligned with the A

• Causes: rotation

EF: on a non fixed object

If an object is without a fixed A, EF would be one where you apply F on the object that is NOT in line with their center of gravity

EF: on a fixed object

If an object has a fixed A an EF would be when you apply F to the object that is NOT on their A.

What is the difference between F and Torque?

Torque is different from F because the result is rotation

• force is linear

• torque is rotation

  • to create torque a force must be eccentric

What is the moment arm/ force arm ?

Perpendicular distance from the line of action of a force and the AoR

The longer the moment arm, the greater…..

The greater the muscular force, the greater….

Which is more efficient?

The torque output would be

The torque output would be

However, manipulating the length of the moment arm is more efficient.

We can’t change levers, but how can we manipulate the moment arm?

We can change the moment arm of a muscle by changing the starting joint angle prior to contraction

Picture ex

At 180 degrees it moves the muscle line of action closer to the joint enter, therefore the moment arm is shorter

Picture ex

At a 90 degree angle the muscles line of action is move further away from the joint center, therefore a bigger moment arm

When we are trying to move the SAME R, would it be easier at a 90 or 180 degree joint angle?

It would be easier at a 90 degree joint angle because less muscular force is used due to the longer moment arm

It would be harder at a 180 degree joint angle because more muscular force is used to the shorter moment arm

What type of system does the patella create? How does it increase the MA of the quadricep muscles?

The patella creates a pulley system, which increase the moment arm of the quads as they are pushed further away from the joint center. Therefore the patella enhances the inherent built in MA for the quads.

We often unconsciously….

This is…..

Change our body/joint position so the moment arm is at its longest.

By increasing the moment arm, we increase torque output and don’t require much muscular input to do the work.

→ This is increasing our leverage, which increases our MA

FA length relative to….

RA length defines how much MA we have

MA in 1st Class Levers

MA is 1 because FA and RA have the potential to be equal

• when MA is 1 it is because the ratio is the same

MA in 2nd Class Levers

• explain how

MA is always > 1 because the FA will always be greater than the RA

• Our muscular contraction force is magnified on a larger FA, so our torque output is relatively larger compared to the muscular force we put into the torque

MA in 3rd Class Levers

MA is always <1 because RA will always be larger than the FA

What formula conceptualizes the relationships between force and resistance components?

F x FA = R x RA

F x FA = R x RA

FA is the distance from the AoR to the line of action which is magnified by the amount of F from the muscle contracting.

R is the amount of R being moved and RA is the distance of the R from the AoR

What is the relationship between the length of 2 lever arms?

There is an inverse relationship between F and FA + R and RA

• The longer the FA the less F is required to move R

  • The shorter the RA, a much larger R can be moved with the same amount of force

Explain

F x FA = R x RA

1 × 5 = 5

5 × 1 = 5

5 = 1 × 5

5 = 5 × 1

If I have a FA of 5 I can only have to input a F of 1 to move a R of 5

If I have a FA of 1 I need to input a (minimum) F of 5 to move a R of 5

If I have a RA of 5 (far from joint center) I can only move something with a R of 1

If I have a RA of 1 (closer to joint center) I can move a much larger R

What is the relationship between force and resistance components?

Proportional relationship between force and resistance components

• If either of the R components increase or decrease, there must be an increase or decrease in one or both force components.

Slight variations in FA length has a huge effect on…

How much F is needed and how many R components we can move.

When the FA and RA length are equal

• resistance….

A force equal to the R is required to balance it

As the length of the FA increases …..

• resistance

A decreasing amount of forces are required to move a relatively larger R

As the FA length decreases…..

• resistance

An increasing amount of F is required to move a relatively small R

What are humans built for

• at the expense of…

We are built for speed and ROM at the expense of F, because we have relatively shorter FA and longer RA

How is our build showcased? Explain it

When an object is rotating around an axis, the points farthest have to move faster than the points closer.

So the longer the RA, the point furthest from the A is moving faster and moves at a larger ROM